1. Field of the Invention
The present invention relates to an organic electroluminescent display device and, more particularly, to a method for arranging a power line at a position where a voltage drop in a cathode electrode and a voltage drop in the power line compensate each other.
2. Description of the Related Art
Generally, an organic electroluminescent display device is a self-emissive display device and may be classified as either a bottom surface emitting structure or a top surface emitting structure according to a direction in which light is emitted from an organic electroluminescent layer. The top surface emitting organic electroluminescent display device is a display device in which light is emitted in a direction opposite to a substrate on which pixels are arranged. With the top surface emitting organic electroluminescent display device, it is possible to increase an aperture ratio, which is an advantage over the bottom surface emitting structure in which light is emitted in a direction of the substrate on which the pixels are arranged.
In the top surface emitting structure, a transparent electrode should be employed as a cathode electrode since light is emitted toward an encapsulation substrate. A transparent conductive film, such as indium tin oxide (ITO) or indium zinc oxide (IZO), is generally employed as the transparent electrode. However, it is difficult to use such transparent electrode as the cathode electrode because of its high work function.
For this reason, as the cathode electrode, a transparent electrode having a laminated structure has been formed by depositing a thin metal with a low work function on an organic electroluminescent layer to form a transflective metal film (i.e. semi-transparent and semi-reflective metal film) and by depositing a thick transparent conductive film on the transflective metal film.
However, in the cathode electrode having the laminated structure, since the organic thin film layer is formed and then the ITO or IZO film (i.e., the transparent conductive film) is deposited thereon, a low temperature vapor deposition process is performed to minimize the deterioration of an electroluminescent (EL) layer due to heat or plasma. Depositing the ITO film or the IZO film at a low temperature causes the quality of the film to be degraded and the resistivity to be increased.
The cathode electrode is a common electrode, which should apply the same voltages to all pixels arranged in a pixel region. However, different levels of voltages are applied to the pixels according to positions of the pixels due to a voltage drop (i.e., IR drop) caused by the high resistivity of the cathode electrode. Consequently, if a cathode voltage from an external terminal is applied to the cathode electrode, a voltage difference is caused between a pixel arranged in a portion adjacent to the external terminal and a pixel arranged in a portion away from the external terminal. It results in non-uniformity of brightness or image quality due to the voltage difference between the pixels based on the positions of the pixels.
In particular, the voltage drop problem in middle and large sized top surface emitting organic electroluminescent display devices is even more serious. In order to solve this problem, a technique using a cathode bus line in a top surface emitting structure as disclosed in Korean Patent No. 2002-0057336 has been developed. Since the cathode bus line is coupled to an external terminal and is in contact with a cathode electrode, the cathode electrode is connected to the external terminal via the cathode bus line.
With such a contact, however, the smaller the portion where the cathode bus line connected to the external terminal and the cathode electrode deposited on an entire surface are contacted, i.e., the area of the cathode contact, the lower the current density, which leads to increase in generated heat.
Further, for a power line for applying a common power supply voltage VDD to the pixels, there is a problem in that a voltage difference is generated between pixels based on the positions of the pixels due to a voltage drop. Previously, a technique for preventing the voltage drop in the driving power line by connecting a bypass line extending in a direction perpendicular to the driving power line, to the driving power line to prevent the resistance increase was disclosed in Korean Laid-open Patent No. 2001-0014501. However, there is a problem with this method in that such a separate bypass line is required to be formed.